Instead of the periodic or on-demand measurement of heart rate that the Apple Watch can currently do, future devices may provide constant monitoring.
Perhaps the very first health feature of the Apple Watch was its ability to sense heart rate. Five years and six versions on, it's still a key feature — and Apple continues to research how to add blood pressure measurements to it.
A pair of newly-revealed patent applications show that Apple wants to make it possible for the Apple Watch to continually monitor blood pressure. "Electrical Coupling of Pulse Transit Time (PTT) Measurement System to Heart for Blood Pressure Measurement," says that it can be done without interruption to the user.
"Current ambulatory and home blood pressure measurement approaches... fail to provide continuous measurement of blood pressure," it says. "Additionally, when an oscillometric blood pressure measurement cuff is used to monitor a person's blood pressure when sleeping, the intermittent inflation and deflation of the cuff can disturb the person's sleeping pattern, thereby harming the subject to some extent and potentially changing the person's sleeping blood pressure."
"Thus, convenient and effective approaches for noninvasive continuous measurement of blood pressure remain of interest," it continues.
Both this, and the similar "Blood Pressure Monitoring Using a Multi-Function Wrist-Worn Device" patent application, propose using the Watch as just one part of the process. In each of these new applications, the Watch will measure blood pressure by timing how long it takes a pulse to reach it from the user's heart.
"Output from a pulse arrival sensor coupled to the wrist-worn device is processed to detect when a blood pressure pulse generated by ejection of the volume of blood from the left ventricle arrives at the wrist," says the first patent application.
"A pulse transit time (PTT) for transit of the blood pressure pulse from the left ventricle to the wrist is calculated," it continues.
This does mean the user wearing electrodes or some other kind of sensor as well as the Apple Watch. So it's not going to be adopted by everyone, it will be used when medical advice recommends continuous study.
"Home blood pressure measurements may be recommended where information is desired regarding the effectiveness of blood pressure lowering medication," explains Apple, "and/or where doubt exists as to the reliability of ambulatory blood pressure measurement."
Apple has previously investigated doing continuous heart rate measures without the use of extras such as electrodes, or a blood pressure cuff.